The present invention relates broadly to proportional pressure control valves such as for use in the control circuits of electro-hydraulic transmissions having hydraulically operated clutches or other.
As is further described in commonly-assigned U.S. Pat. Nos. 6,571,828; 6,405,746; 6,286,535; and 5,836,335, and U.S. Publ. Pat. Appln. No. 2003/0188788, automobiles, trucks, tractors, earth-movers, and other vehicles (hereinafter collectively referred to as vehicles) frequently include an internal combustion engine for powering their movement across the earth's surface. Such vehicles also includes a drive train for transmitting energy produced by the internal combustion engine into movement of the wheels, drive tracks or similar means by which the vehicle is driven across the earth's surface. To effectively accommodate the power characteristics of the internal combustion engine to the load of the vehicle that it must drive at various speeds over varying terrain, an vehicle's drive train usually includes one or more transmissions. Each transmission in an vehicle includes a transmission power input shaft that receives energy from the internal combustion engine's power output shaft, and a transmission power output shaft for transmitting the engine's energy onto the means for driving the vehicle across the earth's surface. Each transmission in an vehicle also includes sets of gears, each one of which, when selected for coupling the transmission's power input shaft to its power output shaft, provides a different speed ratio between the rotation rates, respectively, of the transmission's power input and power output shafts.
To facilitate selecting a particular gear ratio and for smoothly accelerating an vehicle from a stationary start, its drive train usually includes a clutch located between the vehicle's internal combustion engine and its transmission(s). This clutch selectively couples the internal combustion engine's power output shaft to the transmission's power input shaft. In one position of the clutch, it completely decouples the engine's power output shaft from the transmission's power input shaft. In another position, the clutch of an vehicle provides a tight coupling between the internal combustion engine's power output shaft and the transmission's power input shaft. In this tightly coupled state, the internal combustion engine's power output shaft and the transmission's power input shaft rotate at the same speed. However, most clutches for vehicles operating in this tightly coupled state are capable of passing only some maximum amount of torque from the internal combustion engine to the transmission without slippage occurring in the clutch. If a torque greater than this maximum amount is supplied to the clutch in its tightly coupled state, slippage occurs within the clutch that allows the power output shaft of the internal combustion engine to rotate at a speed different from that of the transmission's power input shaft.
Between these two extremes of clutch operation, either of being decoupled or of being tightly coupled, the design of most clutches used in vehicles permit progressively varying the tightness of coupling between the engine's power output shaft and the transmission's power input shaft. In intermediate states between these two extremes, the clutch will transmit an amount of torque to the transmission without slippage that is less than the maximum amount that it will transmit when tightly coupled. Controllably coupling differing amounts of torque from the internal combustion engine to the means for driving the vehicle across the earth's surface permits smoothly accelerating an vehicle into motion. Controllably coupling different amounts of torque from the internal combustion engine to the means for driving the vehicle through the clutch is also useful, particularly for heavy industrial vehicles such as trucks, tractors and the like when shifting the transmission smoothly from a set of gears having one ratio to another set having a different ratio.
Historically, a driver of an vehicle usually operated its clutch through a direct mechanical linkage between the clutch and a clutch pedal located in the vehicle's passenger compartment near the driver. In some instances, a closed hydraulic system for operating the clutch by pressure on the clutch pedal replaces the direct mechanical linkage. More recently, to provide automatic electronic control of gear ratio selection, particularly in vehicle's that include a microprocessor, it has become desirable to control clutch operation by means of an electrical signal rather than by the driver pressing on a clutch pedal. While some designs for clutches are known that permit an electrical current to directly effect coupling and uncoupling of the clutch, such clutches generally consume, and must therefore also dissipate, a significant amount of electrical power. Thus, even with microprocessor controlled operation of an vehicle's transmission, it still appears desirable to continue controlling clutch operation indirectly by converting a control electrical signal from the microprocessor into a more powerful mechanical driving force for directly operating a conventional clutch.
In pursuing this indirect electronic control of vehicle clutches, some vehicle manufacturers have chosen to employ electro-hydraulic transmissions having hydraulically operated clutches. In such electro-hydraulic transmissions, a hydraulic pump supplies pressurized hydraulic fluid for energizing a hydraulic actuator, for example a piston or a bellows, that directly operates the clutch. In one design for such a clutch, springs hold the clutch in its disengaged position and a carefully controlled pressure of the hydraulic fluid from the pump overcomes the springs' force to effect engagement of the clutch. When the hydraulic pressure is removed from this clutch, the springs once again move the clutch into its disengaged state. By using the spring pressure to effect clutch disengagement and hydraulic pressure to effect clutch engagement, the clutch inherently disconnects the engine from the transmission when the engine is not running to power the hydraulic fluid pump. Furthermore, this method of operating an electro-hydraulic clutch inherently avoids creating a hazardous condition if the hydraulic fluid pump fails. With such an electro-hydraulically operated clutch, smoothly accelerating the vehicle into motion and smoothly shifting transmission gear ratios require a hydraulic valve that controls the pressure of the hydraulic fluid supplied to the clutch precisely in response to changing values of the controlling electrical signal.
Solenoid-piloted proportional pressure control valves of the type herein involved for use in electro-hydraulic transmissions are sold commercially by the Integrated Hydraulics Division North America of Parker Hannifin Corporation, Lincolnshire, Ill. These valves and others have proved satisfactory for a diversity of uses, and heretofore have constituted the state of the art. It is believed, however, that further developments in the construction and operation of such valves would be well-received by industry. In this regard, it a believed a preferred construction for certain applications would allow for stable operation even at high flow rates.